Technical Field
The invention relates generally to precision measurement instruments, and particularly to absolute position encoders.
Description of the Related Art
Various position transducers are available, such as optical, capacitive, and inductive transducers. These transducers may use a transmitter and a receiver in a read head to measure its movement relative to a scale. Some types of transducers are sensitive to contamination, such that using them in manufacturing or shop environments is impractical. In contrast, inductive sensors may be immune to contamination by particles, oil, water, and other fluids. U.S. Pat. No. 6,011,389 (the '389 patent), which is hereby incorporated herein by reference in its entirety, describes an induced current position transducer usable in high accuracy applications. U.S. Pat. No. 5,973,494 (the '494 patent) and U.S. Pat. No. 6,002,250 (the '250 patent), which are each hereby incorporated herein by reference in their entireties, describe incremental position inductive calipers and linear scales, including signal generating and processing circuits. U.S. Pat. Nos. 5,886,519, 5,841,274, and 5,894,678, which are each hereby incorporated herein by reference in their entireties, describe absolute position inductive calipers and electronic tape measures using the induced current transducer. As described in these patents, the induced current transducer may be readily manufactured using known printed circuit board technology.
Different implementations of the induced current transducer (and other types of transducers) may be implemented as either incremental or absolute position encoders. In general, incremental position encoders utilize a scale that allows the displacement of a read head relative to a scale to be determined by accumulating incremental units of displacement, starting from an initial point along the scale. However, in certain applications, such as those where encoders are used in low power consumption devices, it is more desirable to use absolute position encoders. Absolute position encoders provide a unique output signal, or combination of signals, at each position (of a read head) along a scale. They do not require continuous accumulation of incremental displacements in order to identify a position. Thus, absolute position encoders allow various power conservation schemes, amongst other advantages.
U.S. Pat. No. 6,034,624 discloses a scale with two periodic pattern components with two different wavelengths which may be suitable for an absolute position encoder. However, a detector disclosed in this reference may be subject to poor rotational misalignment tolerances since its receiver coils do not extend around the entire scale. Furthermore, if the receiver coils did extend around the entire track, the receiver coils would not be able to separate signal content corresponding to each of the two wavelengths. Improved configurations of absolute encoders that are robust against alignment errors would be desirable.